1. Field of the Invention
The present invention relates to spectrally selective optical systems; and more particularly, spectrally-selective optical systems employing 1-dimensional dispersive elements (such as a grating).
2. Description of Related Art
Fiber optic networks employing wavelength-division multiplexing (WDM) require various devices capable of changing fiber mode characteristics, or the data encoded on those modes, in a distinct way for each wavelength “channel” co-propagating on that mode. These changes might include creating loss in a channel, re-routing channels from a first fiber to other fibers, or other treatments such as dispersion correction, detection and diagnostics. Generally, these functions require separation of co-propagating channels (demultiplexing or “demux”) and recombination of those channels (multiplexing or “mux”) after wavelength-specific conditioning is done.
Coupling the fiber mode into free-space allows access to these channels for treatment by “bulk” elements like lenses and gratings, and wavelength-specific treatment of beams using devices such as MEMS tilt mirrors or interferometers, or alternate devices like liquid crystal modulators. In many cases, such as in the use of a grating, the channel wavelengths are dispersed in a single direction such that the position (or angle) of the beams containing all wavelengths lie nominally in a common plane. This is the property of a 1-dimensional (1-D) dispersing element such as a regular parallel-line grating. In these cases, an array of devices that act on the individual wavelengths would be aligned to the array of beams. An example of this is shown in FIG. 1, where light from optical fiber(s) 10 passes through a condenser lens 12 and is separated into beams of particular wavelengths by a grating 14. The separated beams pass through the condenser lens 12 and are incident on an active linear device 16. The separated beams reflected back through the condenser lens 12 are rejoined by the grating 14 and re-enter the fiber(s) 10 after passing through the condenser lens 12.
It is common to define the axis of an optical system as “z” and imagine beams or rays progressing reasonably parallel to it such that this axis defines the sequence of changes to the beam as elements are traversed. For the system in FIG. 1, the z axis proceeds left to right in the page and is centered on the rotationally symmetric condenser lens 12. For systems not having a 1-D dispersive element like the grating, the remaining x and y axis definitions may be arbitrary, especially those systems described as “morphic”. These are generally systems that use substantially spherical or rotationally symmetric surface elements and preserve the basic shape of the beam. For a single wavelength in FIG. 1, the round fiber mode is imaged as a round mode onto the active device 16, and then re-imaged back to itself as a round mode. The cross-sectional footprint of light at the grating location is round even though the grating tilt may project this footprint into an ellipse. For one wavelength and appropriate grating tilt, the effective rotational symmetry of the system in FIG. 1 is preserved.
When the wavelengths of interest are considered, the grating 14 or other 1-D dispersive element breaks the rotational symmetry of the system and defines the distinct non-dispersive (x) and dispersive (y) axes. The spots formed on the active device 16 are arrayed along the y axis according to wavelength. In addition, the spectral resolution of the system—the ability to distinguish spots of different wavelength and separately act on them—is improved if the spot sizes are small because two separable spots can then correspond to nearer wavelengths. This spot size is determined primarily by the convergence angle of the beam approaching that spot, or equivalently the optics numerical aperture (NA), if the system is diffraction limited as needed for efficient fiber coupling. These distinct co-existing system and axis definitions are shown in FIG. 2.
As FIG. 2 indicates, the y-z system must have a large enough NA to provide for adequate convergence angle for the wavelengths of interest, plus a wide enough well-corrected field for the active device that interacts with the dispersed spots. This combined large NA and field size requirement over a substantial wavelength range leads to complex optical designs employing adequate aberration correction and telecentricity. This requirement can lead to cost or optical loss disadvantages. It may be desirable to avoid enlarging the NA or field size requirements as functional or performance enhancements are considered.